drm/modeset-helper: Add simple modeset suspend/resume helpers

Add drm_mode_config_helper_suspend/resume() which takes care of
atomic modeset suspend/resume for simple use cases.
The suspend state is stored in struct drm_mode_config.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171106191812.38927-3-noralf@tronnes.org